It appears portals have not been included in UE4. Several attempts have been made to replicate the look.
This never fully worked in UDK3 for the Oculus Rift (the right eye didn't render). I would love to add portals into my maps but it looks it is going to be a while before that materializes.
I'm sure they are avoiding it because it is a performance nightmare.
Web pages on the topic:
Demo of a hacked portal:
Here is the UE4 roadmap (no portals there):